The cementing process is one of the most important processes in drilling and completing a well. It is an intimate part of the running of casing. Cementing is done at various points in the well and at various times while drilling both inside and outside of the casing.
The primary cementing can form a protective sheath around the casing, segregating producing formations to prevent migration of undesirable fluids. Secondary cementing takes place after the primary cementing and can be used to squeeze cement into the perforations in the casing or to seal off, isolate or repair parts of the well. Plug back cementing is used to place cement at desired points in the well or to shut off the bottom water or reduce the depth of the well.
Two of the apparatuses or pipe attachments routinely used in the cementing operations are the collar and the shoe. These are typically cement restrictions or shoulders which are attached to a pipe string as a part of the pipe string. The collar, for example a float collar, is inserted between the top and bottom of a casing string usually one or two joints above a float shoe which is attached to the bottom of a pipe string. Shoes and collars, among a number of things, help prevent the back flow of cement during the cementing operation. The collars and the shoes are usually equipped with a check valve (often a ball valve means) to aid in the prevention of back flow of cement. The shoes and collars are typically an outer cylindrical housing or pipe and an inner cement tube communicating with and fixed to the inner surface of the cylindrical housing, with a fluid passage running the length of the cement tube. When there is a check valve, it is usually part of an inner housing in concentric spaced relationship with the outer housing so that the cement tube fills the space between the two housings and the inner housing forms part of the fluid passage.
In addition to the collars and shoes typical cementing operations employ one or more pump down plugs. Pump down wipe plugs can serve three purposes: (1) to separate or serve as the interface between the wet cement from the fluid it is displacing or the fluid which is being used to pump the wet cement to the desired level; (2) to wipe off the inner surface of the pipe string as it passes; and (3) to help prevent back flow while the cement is setting up.
In practice the well operator makes up his pipe string so that the collar or shoe is lowered into the well to the desired level. When he decides to cement he may place a bottom pump down wipe plug between the fluid already in the well and the wet cement. This bottom plug has a fluid passage through it which is sealed by a diaphragm or membrane. The cement is pumped into the well forcing the bottom plug down the well, displacing the fluid in front of it, until it reaches the top of the cement tube of the shoe or collar or shoulder. This restriction stops the plug and increased pumping pressure breaks the diaphragm or membrane and the cement passes through the plug and through the fluid passage of the collar or shoe. After the desired amount of cement is pumped into the well a top pump down wipe plug is inserted to act as the interface between the fluid used to force the cement to the desired level for the cement. Often the bottom plug is not used and only one plug as the interface between the cement and the fluid used to force the cement to the desired level is used. The top plug is usually pumped until it comes in contact with the bottom plug if one is used or the top of the cement tube part of the shoe or collar. The cement is allowed to set or harden and the well operator then carries out whatever other operations he intends to do.
The plugs used in the above operation are usually made of a pliable or rubbery material, such as plastic, wood or rubber, sometimes with hollow metal or plastic cores and they fit snugly in the pipe string. All of the plug is made of drillable material.
Once the cement has set up and the well operator has carried out his desired operations he may decide to drill out the plug and/or plugs, collar or shoe and the cement. The plugs are typically made of drillable material, as are the cement tube and and innerhousing of the collar and shoe and of course, the cement which was pumped into the well. The well operator lowers the drill string into the well until the drill bit contacts the plug and he begins to drill by rotating the drill bit, usually clcokwise. In many instances the rotation of the drill bit will cause the plug with which it is in contact to rotate, slipping over the surface on which it rests, i.e., cement, a bottom plug or the cement tube of the shoe or collar. This tendency of the plug to rotate as the bit rotates, to slide across the surface below it, wastes both time and energy. Since all of the components are made of readily drillable material, this wasting of time and energy in the drilling process is an unnecessary problem.
The present invention is an apparatus to be placed between a pump down wipe plug and a cement surface or between two plugs within a well in order to enhance the drillability of the plugs and an improved process for cementing with and drilling through a pump down wipe plug.